This invention relates to a passive low pass filter and more particularly to a passive low pass, xcfx80-type, filter of a surface mount device configuration.
Low pass filters are circuits used in computer, communications, and automotive industries. Currently, discrete components such as ferrite block and surface mounted device, SMD, capacitors are used as low pass filters to reduce noise generated by nearby electrical components. The filters reduce or attenuate noise induced frequencies which lie outside a preferred filter pass band, while passing electrical signals having frequencies within the specified pass band. Therefore, the passive low pass filter behaves as a frequency sensitive voltage divider. At high frequencies the output behaves as if it is shorted while at low frequencies the output appears as an open circuit.
Passive low pass filters comprise one or more inductances and capacitances tuned to suppress or attenuate the unwanted signals while allowing passage of a band of wanted signals. A typical low pass filter has an inductor, a capacitor and a dielectric encapsulant. The encapsulant can be formed from a variety of non-conducting materials , including dielectrics, ferrites, and dielectric-ferrite composites, and commonly encases the filter and engages to and separates a plurality of plates which comprise the capacitor. A variety of low pass filters of a surface mount configuration exist, they include a T-filter, a LC-filter and a xcfx80-filter.
The consumer markets for low pass filters are requiring greater frequency attenuation utilizing smaller components. The layout of the SMD filter is such that electric and magnetic containment must be substantially maintained within the entire yet downsized volume of the encapsulant. As attenuation increases, noise produced within the filter itself becomes a greater problem. The magnetic field of the inductor, otherwise known as a coil, may produce a form of noise known as eddy currents across the signal plates.
A passive low pass, xcfx80-type, filter of a surface mounted device configuration has two stacked capacitors disposed radially inwardly of a coil. Each capacitor has a number of ground plates successively stacked between a number of signal plates. The ground plates electrically engage a front and rear ground band which circles the external surface of a non-conductive encapsulant of the filter. A front and rear conductive member or sheet engages each end of the encapsulant. One end of the coil is engaged to the front sheet and the opposite end is engaged to the rear sheet. The signal plates of the forward capacitor are engaged to the front sheet and the signal plates of the rearward capacitor are also engaged electrically to the rear sheet. Preferably, a centerline of the coil is disposed perpendicular to and intersects a shared centerline of the front and rear bands.
Thus, advantages of the present invention is a reduced size while maintaining or maximizing power dissipation through the filter, and the elimination of eddy noise-producing currents.